I think this is more about her than you.
As someone who has gone through fertility issues, I have one friend who I particularly struggle with for her being insensitive when I thought she was a better friend than she's turned out to be, I had counted her as one of my best friends.
I personally was finding some of her posts really offensive as they were saying stuff like I couldn't imagine ever not being a mummy, my world would never ever be complete etc., despite her knowing my situation she posted that the same day I'd made the effort to drive over an hour to see her new baby. I made an effort to get her a really personalised gift and never got a thank you everyone else who had, had something personally made she thanked and took pics of for fb/Instagram. When she had her second child again she was totally insensitive, in her post saying oh I'm blessed for having a second baby first time of trying.
Everyone thinks she's wonderful and the most amazing person in the world one of those people everyone loves which makes how she's treated me even harder, as no one believes she would be like that.
She hasn't congratulated me since I found out I was pregnant after fertility treatment, says it all really.
Sorry for the ramble, I think she is being overly sensitive of your use of rainbows, but it may well be that be that she is suffering and really she needs you more than you realise, is what I'm trying to get at. I just wanted a little bit more sensitivity of her and some support rather than lose her as a friend, but sometimes the easiest thing to do is push people away like she is doing.